DNA in eukaryotic cells is tightly complexed with proteins (histones) to form chromatin. Histones are small, positively charged proteins that are rich in basic amino acids (positively charged at physiological pH), which contact the phosphate groups (negatively charged at physiological pH) of DNA. There are five main classes of histones H1, H2A, H2B, H3, and H4. The amino acid sequences of H2A, H2B, H3, and H4 show remarkable conservation between species, wherein H1 varies somewhat and in some cases is replaced by another histone, e.g., H5. Four pairs of each of H2A, H2B, H3 and H4 together form a disk-shaped octomeric protein core, around which DNA (about 140 base pairs) is wound to form a nucleosome. Individual nucleosomes are connected by short stretches of linker DNA associated with another histone molecule to form a structure resembling a beaded string, which is itself arranged in a helical stack, known as a solenoid.
The majority of histones are synthesized during the S phase of the cell cycle, and newly synthesized histones quickly enter the nucleus to become associated with DNA. Within minutes of its synthesis, new DNA becomes associated with histones in nucleosomal structures.
A small fraction of histones, more specifically, the amino acid side chains thereof, are enzymatically modified by post-translational addition of methyl, acetyl, or phosphate groups, neutralizing the positive charge of the side chain, or converting it to a negative charge. For example, lysine and arginine groups may be methylated, lysine groups may be acetylated, and serine groups may be phosphorylated. For lysine, the —(CH2)4—NH2 sidechain may be acetylated, for example by an acetyltransferase enzyme to give the amide —(CH2)4—NHC(═O)CH3. Methylation, acetylation, and phosphorylation of amino termini of histones that extend from the nucleosomal core affect chromatin structure and gene expression. Spencer and Davie 1999. Gene 240:1 1-12.
Acetylation and deacetylation of histones is associated with transcriptional events leading to cell proliferation and/or differentiation. Regulation of the function of transcriptional factors is also mediated through acetylation. Recent reviews on histone deacetylation include Kouzarides et al., 1999, Curr. Opin. Genet. Dev. 9:1, 40-48 and Pazin et al., 1997, 89:3 325-328.
The correlation between acetylation status of histones and the transcription of genes has been known for quite some time. Certain enzymes, specifically acetylases (e.g., histone acetyltransferases (HAT) and deacetylases (histone deacetylases or HDACs), which regulate the acetylation state of histones have been identified in many organisms and have been implicated in the regulation of numerous genes, confirming a link between acetylation and transcription. In general, histone acetylation is believed to correlate with transcriptional activation, whereas histone deacetylation is believed to be associated with gene repression.
A growing number of histone deacetylases (HDACs) have been identified. HDACs function as part of large multiprotein complexes, which are tethered to the promoter and repress transcription. Well characterized transcriptional repressors such as MAD, nuclear receptors and YY1 associate with HDAC complexes to exert their repressor function.
Studies of HDAC inhibitors have shown that these enzymes play an important role in cell proliferation and differentiation. HDACs are believed to be associated with a variety of different disease states including, but not limited to cell proliferative diseases and conditions (Marks, P. A., Richon, V. M., Breslow, R. and Rifkind, R. A., J. Natl. Cancer Inst. (Bethesda) 92, 1210-1215, 2000) such as leukemia (Lin et al., 1998. Nature 391: 811-814; Grignani et al. 1998. Nature 391: 815-818; Warrell et al., 1998, J. Natl. Cancer Inst. 90:1621-1625; Gelmetti et al., 1998, Mol. Cell Biol. 18:7185-7191; Wang et al., 1998, PNAS 951 0860-10865), melanomas/squamous cell carcinomas (Gillenwater et al., 1998, Int. J. Cancer 75217-224; Saunders et al., 1999, Cancer Res. 59:399-404), breast cancer, prostrate cancer, bladder cancer (Gelmetti et al., 1998, Mol. Cell Biol. 18:7185-7191; Wang et al., 1998, PNAS 951 0860-10865), lung cancer, ovarian cancer, colon cancer (Hassig et al., 1997, Chem. Biol. 4:783-789; Archer et al., 1998, PNAS, 956791-6796; Swendeman et al., 1999, Proc. Amer. Assoc. Cancer Res. 40, Abstract #3836), and hyperproliferative skin disease such as cancerous and precancerous skin lesions, as well as inflammatory cutaneous disorders.
Histone deacetylase inhibitors are potent inducers of growth arrest, differentiation, or apoptotic cell death in a variety of transformed cells in culture and in tumor bearing animals (Histone deacetylase inhibitors as new cancer drugs, Marks, P. A., Richon, V. M., Breslow, R. and Rifkind, R. A., Current Opinions in Oncology, 2001, Nov. 13 (6): 477-83; Histone deacetylases and cancer: causes and therapies, Marks, P., Rifkind, R. A., Richon, V. M., Breslow, R., Miller, T. and Kelly, W. K., Nat. Rev. Cancer 2001 Dec. 1 (3):194-202). In addition, HDAC inhibitors are useful in the treatment or prevention of protozoal diseases (U.S. Pat. No. 5,922,837) and psoriasis (PCT Publication No. WO 02/26696).
Accordingly, despite the various HDAC inhibitors that have been reported to date, a need continues to exist for new and more effective inhibitors of HDACs.